1. Field of the Invention
This invention relates to a method for forming a silica protective film, which serves to improve mechanical characteristics and corrosion resistance of the surface of a material, such that the silica protective film may exhibit good film-forming properties under low temperature conditions and may have a high mechanical strength. This invention also relates to a process for producing a magnetic recording medium, which is provided with the silica protective film. This invention further relates to an inorganic protective film, which serves to improve mechanical characteristics and wear resistance of the surface of a material, and a magnetic recording medium provided with the inorganic protective film.
2. Description of the Prior Art
As media for recording magnetic information, such as magnetic tapes and hard disks, magnetic recording media, which are provided with magnetic layers constituted of thin ferromagnetic metal films suitable for high density recording, have heretofore been used widely. The magnetic recording media, which are provided with magnetic layers constituted of thin ferromagnetic metal films, can easily achieve a high level of magnetic energy and a high level of surface smoothness. Therefore, the space between the magnetic recording medium and a magnetic head can be kept small, the spacing loss can be kept small, and good electromagnetic transducing characteristics can be obtained.
However, the wear resistance of the thin ferromagnetic metal film types of magnetic layers is lower than the wear resistance of conventional coated types of magnetic layers. Therefore, the thin ferromagnetic metal film types of magnetic layers have the problems in that they readily wear out due to sliding in contact with magnetic heads, or the like, and cannot have good durability. Accordingly, ordinarily, protective films, which are constituted of inorganic oxides, e.g. silica or zirconia, or carbon, are formed on the thin ferromagnetic metal film types of magnetic layers, and the wear resistance of the magnetic layers is thereby enhanced.
Among the protective films, carbon protective films have recently attracted particular attention. The carbon protective films are formed with a vacuum film forming process, such as a sputtering process or a chemical vapor deposition (CVD) process. Therefore, the carbon protective films have the problems in that they cannot be formed quickly and with a high mass production efficiency.
Also, the protective films formed with the vacuum film forming process cannot have sufficient covering properties. In cases where the substrates, on which the protective film is formed, have a complicated uneven shape, the problems occur in that pinholes readily occur in the protective film. These problems are increased when the thickness of the protective film is thin. With the protective film having a film thickness of not larger than 20 nm, e.g. the protective film overlaid on a magnetic recording medium, the problems occur in that, for example, the corrosion resistance cannot be enhanced to a desired level.
In cases where the protective films, which are constituted of inorganic oxides, are formed with the vacuum film forming process, the same problems as those with the carbon protective films are encountered. These problems can be eliminated by, for example, a technique for forming the inorganic oxide protective films with a sol-gel process. With this technique, the productivity can be enhanced. Also, even if the film thickness is thin, a protective film free from pinholes can be obtained regardless of the shape of the substrate, on which the protective film is formed, by appropriately selecting a coating method. Further, in cases where the sol-gel process is employed, it is possible to form protective films, which are constituted of various kinds of inorganic oxides, such as silica, zirconia, alumina, titania, and combinations of two or more of these oxides.
However, the coating film (i.e., the dry gel film), which is obtained with the sol-gel process by applying a sol composition to a substrate and drying it, is porous. Therefore, such that a sufficiently dense film may be obtained, the coating film must be fired at comparatively high temperatures, and should preferably be heated to a temperature of at least 500.degree. C. Thus the sol-gel process requires heat treatment, which is severe even to hard disks formed on metal or glass substrates. In particular, it is difficult to apply the sol-gel process requiring severe heat treatment to flexible media formed on plastic substrates.
Further, in order for appropriate silica films to be formed with the sol-gel process, it is necessary to use acid catalysts for adjusting the rate of hydrolysis and the rate of polymerization of the starting materials, such as alkoxides. However, in cases where the acid catalysts are used, the problems occur in that the corrosion resistance of the magnetic layer is adversely affected and corrosion of the production apparatus is caused to occur.
Accordingly, it is considered to use a polysilazane capable of being converted into silica by oxidation and to form a silica protective film with a coating process. However, the temperature, at which the polysilazane can be converted into silica by heating and oxidation, is as high as at least 400.degree. C. Even with easy-decomposition types of polysilazanes, the conversion temperature is as high as at least 250.degree. C. Therefore, it was difficult to apply the coating process using polysilazanes to the film formation on substrates, such as plastic substrates, which are apt to be adversely affected by heat.